Topics In This Section
1. Introduction
2. General Rules of Frying
3. Temperature Control
4. Preparation of Food
5. Fat Absorption and Topping Up
6. Choice of Frying Medium
7. The Spoilage of Fats and Oils
8. Cleaning
9. Frying Specific Foods
10. Recommended Frying Temperatures
11. The Basic Chemistry of Fats and Oils
12. The Chemistry of Fat Spoilage
13. The Visible Effects of Chemical Spoilage
14. The Role of the Oil Refiner
15. The Role of the Fryer
16. Fire Hazards
Reproduced with permission from Pura Food Products Limited
7 The Spoilage of Fats and Oils
As a fat or oil is used for frying. Certain changes take place which are mainly caused by the effect of high temperature in the presence of air and water. The oil
becomes darker in colour and more viscous. It will tend to froth particularly when wet foods are fried in it, Smoke is more readily formed. Surfaces of the frying
vessel become coated with a brown resinous material which is difficult to remove. Fried food becomes more discoloured and patchy in colour, and it begins to possess
a poorer flavour and a greasier texture. Off odours will begin to be noticeable.
These are all caused by the natural deterioration of the oil. By the time a frying medium is discarded, probably about one fifth of it will have been altered by these
breakdown processes. The frying life of an oil will depend on the nature of the oil being used and on the way in which it has been treated.
Apart from the normal precautions which have been mentioned in Section 2, the rate of breakdown of an oil will depend upon its exposure to the oxygen in the air.
It is, therefore, dependent on the surface area of the oil exposed to air and to a certain extent the steam which is given off from the food during frying tends to blanket
the oil from oxygen. This suggests that it is better to keep the oil busily frying food for as long as possible. An oil held at frying temperature but standing idle is
rather more likely to deteriorate.
Many metals, notoriously copper, (and its alloys such as brass) and iron tend to accelerate the breakdown of fat. Some types of thermometer are made of brass, and
often drainage taps and 'home-made' pieces of equipment including strainers, ladles and containers, are made from the wrong metal. These should all be avoided.